"Flabby." "Out of breath." "Needs to trim some excess." Theatre critics naturally love using words and phrases like these when a show all but dares their deployment. But sometimes that same show makes it impossible. Such is the case with the musical that just opened at the American Theatre of the Actors. There's no way around it: Fat Camp is big fun.

Randy Blair (book and lyrics), Tim Drucker (book), and Matthew roi Berger (music) have delivered a devilishly tuneful and warmly entertaining evening that fills you up but doesn't pack on the pounds. If it grows a bit svelte on ingenuity after intermission, it's nonetheless an energetic, eclectic corrective for what was, all things considered, a lackluster spring for tuners on Broadway.

There's not much to say about the story that can't be gleaned from the title alone. The getaway in question is Camp Overton, and its charges are high-schoolers of various ages and persuasions, though most share a predictably ample girth. Leading the kids is Robert Grisetti (Daniel Everidge), one of the biggest of all and the least interested in losing any weight. He's more determined to gain the notice of Taylor (Molly Hager), the most committed of the campers, while avoiding the annoying and nasty behavior of Overton all-star Brent Von Bingenberger (Jared Zirilli), who so took to the program the year before that he went from fattie to hottie (he revels in his six pack, and not of the Dr. Pepper variety) in a year and won't let anyone forget it.

Fat Camp typically sticks within the traditional outlines camp-set movies and TV series, even to the point of having a subplot about selling stolen candy and a big competition at the end to see whether or not thin truly is in — originality of plot is not the musical's strong point. Instead it establishes and gains momentum from its charming, but seldom smarmy, lighthearted attitude, which manifests itself in both the joke-packed book and the preternaturally youthful score, which views everything from the seasons to romance to the size of one's posterior as an event worthy of vocalizing about. The big introductory number ("Can't Take Away My Summer"), the Act I closer ("Top of the World"), and the Act II finale ("Feels a Little Bit Like Love") are as joyful as any new song Broadway heard this past year.

Strong, rock-tinged orchestrations, arrangements, and musical direction by Jason DeBord add to the score's sparkle; choreographer Kelley Devine and designers Beowulf Boritt (sets), David C. Woolard (costumes), and Jason Lyons (lights) have smoothly transformed the theater into a sunny world of adolescent whimsy. The performances, too, are top-notch, with Everidge a droll and dynamic lead, Hager a sweet and sympathetic (but never saccharine) heroine, and Zirilli a wryly manic pseudo-villain; better-known names like Marcus Neville and Janet Dickinson, as the two head counselors, and Carly Jibson (best known for her Broadway performances in Hairspray and Cry-Baby), ideal in a tart supporting comic role (the whorish girl, for what it's worth), only enhance the experience.

Since it premiered at the 2009 New York Musical Theatre Festival, the show has only become firmer and more toned, and its cast (many of whom are repeating their roles here) more honestly committed to their work. A few problems remain, mostly in the second act: In too many scenes there, the writing degenerates into meta-theatrical gags that puncture the stylized, pristine silliness Act I works so hard to generate, and the actors can't sell the associated laughs as effortlessly. Should the show move on still further, Blair and Drucker, who are otherwise bull's-eye precise in hitting their comic targets, would be wise to revisit some of these choices. (Although I for one would like to encourage them to keep the curtain call candy toss.)

When Fat Camp is at the top of its game, however, it's a delight — and even something of a thoughtful one. Rare indeed is a musical about staying true to your own beliefs and maintaining a positive self-image that is enacted by performers who actually look like they possess the confidence of individuality they're trying to inspire in others. Don't expect to find much in the way of padding or prosthetics here either and, what's more, don't expect to care: The writing and the acting draw you into these people's concerns, and make you see them for the unique personalities they are. Once you've submitted, everything else vanishes, and the only thing that still seems oversize is the talent on display.